New data shows a worrying trend: Baltimore harbor’s health is in decline

Despite decades of work to clean up Baltimore’s harbor and restore its habitat for fish and other underwater life, new data suggests a concerning trend: Water quality is getting worse.

Testing sites across the harbor — an area that stretches from downtown to the Middle Branch — ranged from “poor” to “fair,” according to 2025 water-quality data published Wednesday by the nonprofit Blue Water Baltimore.

Alice Volpitta, Blue Water’s harbor water-quality watchdog, said more than a decade of monitoring by her organization suggests concentrations of phosphorus, an oxygen-depleting nutrient, are likely to further deplete the harbor ecosystem.

“We can talk all day long about whether ‘fair’ is good enough for the residents of Baltimore,” Volpitta said. “We have to start thinking: ‘Is this good for the next generation or the generation after that?’”

Exactly what’s driving this trend remains something of a mystery, but Volpitta said it shows how much more needs to be done to curb stormwater runoff. Blue Water’s annual report showed a similar trend a year ago, and Volpitta said changes from year to year are small.

Even so, the results come on the heels of a rough stretch for the Baltimore harbor.

Persistent rains last summer dashed plans for the second Harbor Splash event, when some 200 people were registered to jump into the water in Fells Point. As weather cooled at the end of summer, thousands of fish turned up dead in the water while the harbor took on a sickly green shade that lasted for weeks.

Known locally as a “pistachio tide,” this event was triggered by weather conditions but made worse by decades of harbor pollution, harbor scientists say. Last year’s event in September and October was the longest of its kind in recent memory.

Blue Water has monitored water quality in the harbor since 2013. The group’s report focuses on the health of the ecosystem — including underwater life such as fish, crabs and plants — but Volpitta said the news about the water’s safety for human recreation is brighter.

Levels of harmful bacteria such as enterococcus have improved, a sign that efforts to control sewage pollution are paying off. Baltimore’s Department of Public Works has operated under consent decrees to fix its wastewater treatment plants and end sewage overflows, which contaminate city waterways during heavy rains. The department projects it will have spent more than $2 billion on system improvements by the end of the decade.

Adam Lindquist, the vice president of the Waterfront Partnership, is one of Baltimore’s most vocal advocates for a swimmable harbor. His organization plans to bring back organized swims this summer — this time staging events on multiple days — and Lindquist thinks the harbor could soon have a permanent swimming area.

The improving bacteria scores are an encouraging sign, Lindquist said, but he stressed the importance of turning around ecosystem health, too.

Although reviving the Patapsco’s ecosystem has proven frustrating, testing stations in the streams that flow into the harbor show more encouraging results.

Other factors, including climate change and increased rainfall, however, could exacerbate the harbor’s challenges.

Volpitta said the harbor’s middling scores are driven largely by the growing concentration of phosphorus, a nutrient that can drive algal blooms that sap oxygen. What’s causing this phosphorus trend isn’t clear, but she speculated warming waters may be triggering the release of dormant phosphorus pollution at the bottom of the harbor.

“There are processes at work here that we don’t fully understand,” Volpitta said, “but our data is starting to unlock the fuller picture.”